I voted 1GB. For proper industrial flash, I find that 4GB cards are about 3x more expensive than 1GB cards, at around $100 a piece. That's a little too rich for me when the rest of the hardware I'm likely to use has a similar combined cost, this will make my average install about 30% more expensive. The difference between 512MB and 1GB cards is relatively small, but jumping to 2GB almost doubles the 1GB price.
Besides, I see little use for lots of extra storage other than installing a few packages which are probably a few MB each at most. Running a Squid cache or such off CF just doesn't make sense to me.